docs: def Fourar_Bories(x, mul, mug, rhol, rhog):
      r'''Calculates a suggested definition for liquid-gas two-phase flow 
      viscosity in internal pipe flow according to the form in [1]_ and shown
      in [2]_ and [3]_.
          
      .. math::
          \mu_m = \rho_m\left(\sqrt{x\nu_g} + \sqrt{(1-x)\nu_l}\right)^2
          
      Parameters
      ----------
      x : float
          Quality of the gas-liquid flow, [-]
      mul : float
          Viscosity of liquid, [Pa*s]
      mug : float
          Viscosity of gas, [Pa*s]
      rhol : float
          Density of the liquid, [kg/m^3]
      rhog : float
          Density of the gas, [kg/m^3]
  
      Returns
      -------
      mu_lg : float
          Liquid-gas viscosity (**a suggested definition, potentially useful
          for empirical work only!**) [Pa*s]
  
      Notes
      -----
      This model converges to the liquid or gas viscosity as the quality
      approaches either limits.
      
      This was first expressed in the equalivalent form as follows:
          
      .. math::
          \mu_m = \rho_m\left(x\nu_g + (1-x)\nu_l + 2\sqrt{x(1-x)\nu_g\nu_l}
          \right)
      
      Examples
      --------
      >>> Fourar_Bories(x=0.4, mul=1E-3, mug=1E-5, rhol=850, rhog=1.2)
      2.127617150298565e-05
  
      References
      ----------
      .. [1] Fourar, M., and S. Bories. "Experimental Study of Air-Water 
         Two-Phase Flow through a Fracture (Narrow Channel)." International 
         Journal of Multiphase Flow 21, no. 4 (August 1, 1995): 621-37. 
         doi:10.1016/0301-9322(95)00005-I.
      .. [2] Awad, M. M., and Y. S. Muzychka. "Effective Property Models for 
         Homogeneous Two-Phase Flows." Experimental Thermal and Fluid Science 33,
         no. 1 (October 1, 2008): 106-13. 
      .. [3] Aung, NZ, and T. Yuwono. "Evaluation of Mixture Viscosity Models in 
         the Prediction of Two-Phase Flow Pressure Drops." ASEAN Journal on 
         Science and Technology for Development 29, no. 2 (2012). 
      '''
      rhom = 1./(x/rhog + (1. - x)/rhol)
      nul = mul/rhol # = nu_mu_converter(rho=rhol, mu=mul)
      nug = mug/rhog # = nu_mu_converter(rho=rhog, mu=mug)
      return rhom*((x*nug)**0.5 + ((1. - x)*nul)**0.5)**2